1. Field of the Invention
This invention relates to enhanced reflectance infrared energy reflecting compositions for cooking apparatus.
2. Description of the Related Art
Ovens for cooking food have been known and used for thousands of years. One of the simplest and probably the oldest cooking of food resulted when food products were left next to a fire, perhaps on a hot rock, and cooked essentially by a heat transfer method of conduction. With refinement, an enclosure surrounding the heating element entrapped the heated air, giving rise to cooking by convective heat transfer. This process was the prototype for the modern gas or electric oven.
In the past century, radiant energy from energy radiation sources has been used to heat and directly cook foodstuffs. Within the past few decades, microwave ovens have become common, in which microwave radiation cooks the food. This has proved useful in allowing very short cooking times for many types of food.
Ovens using infrared energy sources, for example such as quartz halogen lamps, are used for quick heating of food. These quartz halogen lamp ovens can also be used for cooking, and are common in restaurants. In these ovens most of the heat is infrared energy. This infrared energy is reflected and a majority of the infrared energy is lost into the walls of the oven. The walls of these ovens do not reflect a sufficient amount of infrared energy cooking energy onto the food to be cooked to be an efficient user of energy.
Attempts have been made to line the inside of the quartz halogen lamp ovens with metallized coatings, which are often highly polished coatings. However, the highly polished surfaces cannot withstand the scrubbing and cleaning processes and materials to which ovens are subjected. The cleaning leads to a degradation of the polished metallized coating, and a subsequent reduction in the reflective efficiency of the oven.
Further, the metal reflective surfaces provide only a specular reflectance, and do not efficiently disperse and direct the infrared energy to the food to be heated. The specular reflectance by metallic surfaces provides a direct, "angle in equals angle out" type of reflectance. Thus, the specular reflectance merely reflects around the oven, without a substantial portion of the energy impinging on the food.
Specular reflection of energy from metallic surfaces for use in ovens is known, for example in U.S. Pat. No. 3,304,406 to King; U.S. Pat. No. 4,345,1443 to Craig et al.; U.S. Pat. No. 2,767,297 to Benson; and U.S. Pat. No. 4,455,479 to Itoh et al. These ovens specularly reflect energy, which as discussed above, reflects around the oven, without necessarily all of the energy impinging on the food. The specular reflectance is not dispersed throughout the oven to impinge onto the food. Accordingly, specular reflectance of the infrared energy by polished metal surfaces of an oven is energy inefficient.
It was generally believed that radiation with wavelengths much shorter than 1 micron is not useful in cooking or baking processes, partly because of the weaker interaction of the shorter wavelengths with the foodstuff molecules in terms of general heat transfer, and partly due to the inferior penetrating properties of such radiation. In particular, it was believed that visible light, i.e., radiation with a wavelength in a range between about 0.4 to about 0.7 micron, was not very useful in the cooking process.
However, if a sufficiently intense source of visible light radiation is used with sufficient infrared energy radiation reflection onto the food, an effective cooking apparatus results. The combination of the deeply penetrating reflected infrared radiation and the intense visible radiation establishes a temperature gradient within the interior of the foodstuff that ensures that the surface temperature of the foodstuff is hotter than the interior, and the products of the cooking, i.e., the water vapor and gases like CO.sub.2, are quickly driven to the surface and out of the foodstuff. This process results in a very rapid and efficient cooking of the food.
Using infrared radiation to cook food has a number of significant advantages. The cooking process is very fast. Bakery products, for example, can be baked 5 to 10 times faster than ovens that rely on conventional convection and conduction processes. The quality of the cooking process is enhanced for many foodstuffs. Vegetables are cooked so fast that they are virtually steamed in their own water vapor, leaving them hot, but with very little loss of any of their nutritive values.
The reflectance efficiency of a material composition is dependent on several factors. These factors include the particle size of the reflecting particles and the volume fraction or coverage over the surface of the material composition. An optimum particle size and volume fraction will optimize the reflectance in the desired wavelength. Thus, it is desirable to increase the particle size and increase the volume fraction so as to increase the reflectance of the material composition.
The reflectance efficiency of a certain irradiation wavelength is dependent on three primary factors. These factors are: 1) a difference in the refractive index of the high index scattering particles and the low index surrounding medium, i.e., the higher the difference between the scattering particles and the medium the better; 2) an optimum particle size, typically about 1/3 to 1/2 the subject wavelength; 3) and a volume fraction of the scattering particles, high enough to provide a required number of scatterers optimally spaced within the surrounding medium, thus effecting the refraction, diffraction and reflection.
Enamels contain oxide particles, for example, white enamels Q0808A, XT1056-4, T1363 and XT 1032 of the Ferro Corporation all contain oxides. These enamels comprise a white enamel with at least one of recrystallized and mill added Anatase TiO.sub.2. However, these enamels are not acceptable infrared reflectors because the size and amount or volume fraction of the Anatase TiO.sub.2 particles do not provide a sufficient degree of reflectance. Accordingly, even though these enamels contain Anatase TiO.sub.2, they are not suitable for infrared heating, and there is a significant degree of energy loss.
Anatase TiO.sub.2 is normally precipitated out of the enamel at a firing condition, for example 1500-1550.degree. F. for 3-10 minutes. Anatase TiO.sub.2 has a size less than about 0.5 .mu.m, and normally about 0.2 .mu.m. The state of the art white enamel with Anatase TiO.sub.2 crystals has a reflectivity of about less than .ltoreq.70%. This is not sufficient for efficient heating by infrared energy, since the reflectivity is low and the food will not heat quickly, thus resulting in a waste of energy. At this size, Anatase TiO.sub.2 is optimum for the desirable white color in enamels, caused by the efficient diffuse reflection of the visible spectrum of radiation. However even though an enamel contains Anatase TiO.sub.2, which reflects some heat and has desirable cleansing withstanding characteristics, it will not efficient for reflecting infrared energy to heat food, due to particle size.